1. Field of the Invention
The present invention relates to a mixer circuit which is adapted to mix a plurality of signals at a high S/N (signal-to-noise ratio).
2. Prior Art
Examples of known mixer circuits will be explained referring to the drawings. FIG. 1 is a circuit diagram showing the configuration of one example of known mixer circuit. In FIG. 1, an operational amplifier (op-amp) OP1 has a positive input terminal which is grounded, and a negative input terminal to which input resistors 11-15 are connected in parallel. Feedback resistors 21-26 are connected in series between the output terminal and negative input terminal of the operational amplifier OP1. Also, nodes or junction points between adjacent feedback resistors 21-26 are connected to the output terminal of the operational amplifier OP1 via respective switches SW1-SW5. The input resistors 11-15 have respective resistance values R1-R5, and the feedback resistors 21-26 have respective resistance values R6-R11.
In the mixer circuit constructed as above, input signals V1-V5 (the level of each input signal is measured in voltage V!) are mixed in a proportion corresponding to that of the resistance values R1-R5, and are amplified at the same time. If switch SW3 is in the ON state while all the other switches are in the OFF states, for example, the output signal Vout (the level of which is measured in voltage V!) is given by the following expression (1). EQU Vout=-(R6+R7+R8).multidot.(V1/R1+V2/R2+V3/R3+V4/R4+V5/R5) (1)
When input signals V1-V5 having large amplitude levels are applied to the input terminal of the operating amplifier OP1, the output signal Vout exceeds the dynamic range of the amplifier OP1, and clipping may occur in the output waveform. The switches SW1-SW5 are provided for preventing waveform distortion due to such clipping. In the case where clipping of the output waveform occurs as described above, the switch SW2 is placed in the ON state, and all the other switches are placed in the OFF states. In this case, the output signal Vout is given by the following expression (2) . EQU Vout=-(R630 R7) (V1/R1+V2/R2+V3/R3+V4/R4+V5/R5) (2)
Thus, the level of the output signal Vout obtained when only the switch SW2 is ON is (R6+R7)/(R6+R7+R8) times as high as that of the output signal Vout obtained when only the switch SW3 is ON. In this manner, clipping of the output waveform can be prevented by suitably switching the operating states of the switches SW1-SW5.
In the meantime, the output signal Vout of the mixer circuit is often supplied to a circuit (not shown) in the next stage via wiring, and subjected to various types of processing. In this case, it is desirable in view of the S/N of the whole circuit system that the output signal Vout of the mixer circuit be at the maximum level with no waveform distortion. In some cases, the gain of the mixer circuit is desired to be slightly or minutely changed according to a change in the gain of the circuit in the next stage. To this end, the feedback resistance of the mixer circuit needs to be changed in multiple steps. In this known example, the selection of the gain is made in only six steps, namely, the gain is selected from only six values, by placing a selected one of the switches SW1-SW5 in the ON-state, or placing all of the switches in the OFF state. For example, the same gain is obtained when the switches SW1 and SW5 are in the ON state and the other switches are in the OFF states, and when only the switch SW1 is in the ON state. In order to allow finer or subtler adjustment of the gain, therefore, an increased number of feedback resistors and switches need to be installed on the circuit, resulting in an undesirably increased size of the circuit.
A mixer circuit shown in FIG. 2 is an improvement of the mixer circuit of FIG. 1 in terms of the above-described problem. This mixer circuit is comprised of a mixer portion A and an amplifier portion B. The output signal Va of the mixer portion A is given by the following expression (3). EQU Va=-R6.multidot.(V1/R1+V2/R2+V3/R3+V4/R4+V5/R5) (3)
When only the switch SW2 is placed in the ON state, the output signal Vout of the amplifier portion B is given by the following expression (4). EQU Vout=Va.multidot.(R9+R10+R11+R12)/(R7+R8) (4)
When the switch SW2 and the switch SW4 are placed in the ON states, and the other switches are placed in the OFF states, the output signal Vout of the amplifier portion B is given by the following expression (5). EQU Vout=(R11+R12)/(R7+R8) (5)
In this manner, the gain of the mixer circuit can be changed. In this example, the selection of the gain is made in fifteen different steps, namely, the gain is selected from fifteen values, by placing only one of the switches SW1-SW5 in the ON state, or placing two of the switches SW1-SW5 in the ON state at the same time. Thus, the gain of this mixer circuit can be more finely adjusted as compared with that of the mixer circuit shown in FIG. 1.
In the known mixer circuit shown in FIG. 2, the gain of the mixing portion A is fixed. If the gain of 1the mixer portion A is set to a large value assuming that the amplitude levels of the input signals V1-V5 are small, its output signal Va exceeds the dynamic range of the mixer portion A when large amplitude levels of input signals V1-V5 are applied to this portion A. In this case, the waveform of the output signal Va is clipped. If the gain of the mixer portion A is set to a small value assuming that the amplitude levels of the input signals V1-V5 are large, on the other hand, the amplitude level of the output signal Va becomes small when small amplitude levels of input signal V1-V5 are applied to the mixer portion A. In this case, if noise is introduced between the output of the mixer portion A and the negative input terminal of the operational amplifier OP2, the S/N is deteriorated or reduced due to a small signal component (low level of the output signal Va). Thus, in the known mixer circuit, the waveform distortion and the S/N, which have a trade-off relationship, cannot be both improved at the same time.